Analysis of blood samples often requires separation of whole blood into a serum fraction and a cell-containing fraction. It is well known in the art that whole blood separation can be carried out through centrifugation by disposing whole blood into a blood collection tube, placing the tube into a centrifuge, and spinning down the blood.
Unfortunately, once the blood separates, the fractions of the whole blood can remix causing contamination of the fractions through diffusion, agitation, sample extraction, or other undesirable interaction. Ideally, the two fractions should remain isolated to ensure no contamination occurs when accessing the desired fraction. Furthermore, the analytes of the blood should maintain stability after separation over extended periods of time to provide for storage, shipping, or late term analysis.
Any system that isolates the fractions of whole blood must include a separator substance having a suitable density within the tube. Suitable densities are about 1.04 g/cm3 and are between the density of the heavier cell-containing phase and the density of the lighter serum-containing phase. When whole blood is added to the tube and the tube is centrifuged, the separator substance migrates to between the fractions isolating the two fractions from each other. An example collection tube using a gel as a separator substance and that is flowable with whole blood can be found in U.S. Pat. No. 4,946,601 to Fiehler. An example separator substance that is also flowable with whole blood can be found in U.S. Pat. No. 6,248,844 and U.S. Pat. No. 6,361,700 to Gates et. al. In those patents the substance is a polyester curable to a desired viscosity.
Although providing a flowable substance allows for separating the fractions of whole blood, flowable substances have several disadvantages. A flowable substance remains flowable even after centrifugation which results in a risk of contamination of the sample if proper care is not taken to keep the sample suitably still and protected from agitation. For example, it is known to use a thixotropic gel in a blood collection tube where the gel can still flow after centrifugation. Additionally, known substances lack the ability to maintain analytes (e.g., potassium and glucose) at acceptable levels over extended periods of time (e.g., for at least three days).
U.S. Pat. No. 4,818,418 to Saunders discusses the use of a thixotropic gel in blood collection tubes. The problem with thixotropic gels, however, is they do not form a sufficiently permanent separation barrier between the fractions of whole blood. When a sample is extracted from the tube with a pipette, the substance can contaminate or plug the pipette if it touches the substance due to the flowable nature of the substance. If the substance is formulated or configured with a high viscosity to provide a sufficiently solid or permanent barrier to overcome the previous disadvantages, then the substance is no longer suitably flowable with whole blood resulting in prohibitive centrifuge times. Short centrifuge times are critical in life or death situations where a blood analysis result is required quickly.
An alternative approach taken by collection tube manufactures is to provide moveable solid barriers. Examples of suitable solid substances include the intermediate density polymers found in U.S. Pat. No. 3,647,070 where polymer spheres form the barrier layer. U.S. Pat. No. 5,266,199 describes a tube-and-ball valve that controls separation of the serum from the cell-containing phase. However, such physical barriers do not provide a sufficient seal between the fractions and are often either incomplete and tend to leak, or impracticable for other various reasons.
These and other solutions for whole blood separation lack the necessary features to ensure the separated factions of whole blood are effectively protected against contamination due to undesirable sample interactions while supporting short centrifugation times. Furthermore, known separation technologies fail to maintain stability of analytes, especially potassium and glucose, over extended periods of time. Thus, there is still a need for liquid separation technologies in which the separation layer can be hardened and preserve stability of analytes.